The work in progress includes studies of: 1) the properties of HDL recombinants containing cholesterol, 2) the interactions of phospholipid and cholesterol/phospholipid vesicles with plasma HDL in vitro and in vivo, and 3) the removal of cholesterol from erythrocyte membranes using lipoprotein substrates of defined structure. The principal findings in these respective areas have been: 1) The formation of HDL recombinants from cholesterol/phospholipid liposomes is markedly inhibited at higher cholesterol/phospholipid ratios and the recombinants have a decreased cholesterol content compared to the liposomes. 2) The presence of unesterified cholesterol in their incorporation into plasma HDL, both during in vitro incubation of vesicles with HDL and following injection of vesicles into the rat. 3) Discoidal recombinants of dimyristoyl lecithin/apoA-I remove 3H-cholesterol from erythrocyte membrane more rapidly than vesicles of dimyristoyl lecithin.